Calculus (3rd Edition)

Published by W. H. Freeman
ISBN 10: 1464125260
ISBN 13: 978-1-46412-526-3

Chapter 7 - Exponential Functions - 7.1 Derivative of f(x)=bx and the Number e - Exercises - Page 327: 51

Answer

$t = 1$ corresponds to neither a maximum nor a minimum.

Work Step by Step

Given $$ g(t)=\frac{e^{t}}{t^{2}+1} $$ Since \begin{align*} g'(x) &= \frac{\frac{d}{dt}\left(e^t\right)\left(t^2+1\right)-\frac{d}{dt}\left(t^2+1\right)e^t}{\left(t^2+1\right)^2}\\ &= \frac{e^t\left(t^2+1\right)-2te^t}{\left(t^2+1\right)^2} \end{align*} Then $f(x)$ has critical points when \begin{align*} g'(x)&=0\\ \frac{e^t\left(t^2+1\right)-2te^t}{\left(t^2+1\right)^2} &=0\\ e^t[\left(t^2+1\right)-2t]&=0\\ t^2-2t+1&=0 \end{align*} Then $t=1$ is a critical point; to find the interval of increasing and decreasing regions, choose $t=0,\ t=2$ \begin{align*} g'(0)&>0 \\ g'(2)& >0 \end{align*} Hence, $g'(t) $ does not change sign and therefore $t = 1$ corresponds to neither a maximum nor a minimum.
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